Datasheet

LMC6044
SNOS612D –NOVEMBER 1994–REVISED MARCH 2013 www.ti.com
Figure 30. Example of Guard Ring in P.C. Board Layout
To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC6044's inputs
and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to the op-amp's
inputs, as in Figure 30. To have a significant effect, guard rings should be placed on both the top and bottom of
the PC board. This PC foil must then be connected to a voltage which is at the same voltage as the amplifer
inputs, since no leakage current can flow between two points at the same potential. For example, a PC board
trace-to-pad resistance of 10
12
Ω, which is normally considered a very large resistance, could leak 5 pA if the
trace were a 5V bus adjacent to the pad of the input. This would cause a 100 times degradation from the
LMC6044's actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a
resistance of 10
11
Ω would cause only 0.05 pA of leakage current. See Figure 33 for typical connections of guard
rings for standard op-amp configurations.
Figure 31. Inverting Amplifier Typical Connections of Guard Rings
Figure 32. Non-Inverting Amplifier Typical Connections of Guard Rings
Figure 33. Follower Typical Connections of Guard Rings
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